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OFDM Visible Light Wireless Communication Based on White LEDs

H. Elgala, R. Mesleh, H. Haas and B. Pricope
School of Engineering and Science International University Bremen 28759 Bremen, Germany {h.elgala, r.mesleh, h.haas, & b.pricope}@iu-bremen.de
Abstract— White LEDs are set to penetrate many areas of everyday life. An interesting property of these devices (in addition to their lightening capabilities) is that they can be utilised for data transmission. In the past, primarily OOK (on-off keying) has been used for digital data modulation of such devices. OOK imposes limitations on the achievable data rates. Therefore, in this paper OFDM is considered in combination with higher order modulation schemes. A hardware demonstrator with an entire link chain (transmitter and receiver) is developed and measured BER (bit error ratio) results are reported. The system uses pilot sub-carriers to correct frequency synchronisation errors, training sequences for channel estimation and time synchronisation routines. Forward error correction (FEC) coding is used. It will be shown that for COFDM (coded OFDM) with QPSK (quadrature phase shift keying) modulation and a single LED, a BER of 2 × 10−5 is achieved for a distance of 90cm between transmitter and receiver.

I. I NTRODUCTION In the 21th century, high speed data transmission will play an important role in our daily life. Multimedia information is envisaged to be available at any place and at any time. Wireless access networks constitute a key element to achieving these goals. However, radio frequency bandwidth at frequency ranges which allow reasonable spatial coverage is a limiting factor. Therefore, alternative wireless transmission means have to be explored. Visible light communication using white LEDs offers the potential for such alternative. The main reasons are as follows: • White LEDs are currently penetrating many areas of our everyday life. They are envisaged to replace high energy consuming light bulbs in private and business homes and even in street lamps. Moreover, they can be used in headlights of planes and trains, front and back lights in cars and trains, and for object illumination in museums, etc.. • Bandwidth is not limited. • Existing local powerline infrastructure can potentially be utilised. • Transmitters and receivers devices are cheap, and there is no need for expensive RF units. • As lightwaves do not penetrated opaque objects, they can not be eavesdropped. It is very difﬁcult for an intruder to (covertly) pick up the signal from outside the room. • Visible light radiations are undoubtedly free of any health concerns. Therefore, these systems will receive accep-

tance for use in hospitals, private homes, etc.. Furthermore, no interference with RF based systems exist, so that the use in airplanes is uncritical. Infrared signals were investigated for wireless data transmission, for example, applied to indoor wireless local area networks [1–6]. The wavelengths of infrared and visible light sources are close to each other, and the signals, therefore, qualitatively exhibit a similar propagation behaviour. As a consequence, white LEDs have started to attract attention for use as a data communication means [7, 8]. In addition, white LEDs can offer very high brightness, very low power consumptions and long lifetime. Therefore, a unique feature of white LEDs is that they can serve two purposes at the same time: lighting and high speed wireless data transmission. In addition, unlike infrared transmission, there are no health regulations to restrict the transmit power. The optical medium can be viewed as complementary to the radio medium rather than competitive. For example, if a WLAN (Wireless Local Area Network) is required to cover a large area, where users can roam freely inside and outside a building and remain connected to the network at all times, then radio transmission is the best choice to achieve this. If, however, a WLAN is required to cover a relatively small area, and the service is provided locally inside a room, but high transmission rates are required such as for video conference, digital TV (television) or video on demand, then the optical transmission with almost unlimited bandwidth can be used. At the same time, this would free radio frequency spectrum for other purposes as described above. There are many promising indoor applications including the transmission of television and multimedia signals using the ceiling lamp or the desk lamp, the use of LED light spots in cars, trains, buses and airplanes as Internet access points and the realisation of local information points in shops, airports, train stations and museums. Beside the indoor applications, there are many outdoor applications possible including car to car communication via the front and back lights, trafﬁc lights providing the drivers with trafﬁc related information, and street lamps providing the pedestrians with local information. This paper is organised as follows. In Section II, OFDM is introduced and the advantages over single channel systems are discussed. The visible light data transmission prototype is introduced in Section III. The proposed system model is de-

the FFT (fast Fourier transform) is used to demultiplex the parallel data streams. The electrical signal is then passed through an A/D (analogue/digital) converter followed by cyclic preﬁx elimination and OFDM demodulation. Therefore. Finally. The circuit employs a P (positive) on N (negative) silicon planar photodiode designed to deliver a maximum response
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. The human eye would not be able to follow these variations. In an OFDM system. green. The inherent robustness of OFDM against multipath effects. OFDM is used in the IEEE 802. It can be shown. a guard interval (GI) with a cyclic preﬁx is introduced to preserve the orthogonality between subchannels. also referred to as OFDMA [9]. The orthogonality is achieved by selecting a special equidistant set of discrete carrier frequencies. The processor runs at 250MHz. and blue. PPM (pulse position modulation). FDMA and CDMA (code division multiple access). an analogue circuit with a photodiode is used to convert the optical signal to an electrical signal. These operations are preceded by a frame synchronisation routine. one for the transmitter (Tx) and one for receiver (Rx). a high data rate serial data stream is split up into a set of low rate sub-streams. As a consequence. hence. An open issue is the high PAR (peak-to-average ratio) of the OFDM signal. Some are fabricated using a blue LED chip and a phosphor. OOK. III. channel distortions introduce ISI potentially violating the orthogonality. The parallel data transmission offers possibilities for alleviating many of the problems encountered with serial transmission systems [10] such as intersymbol interference (ISI) and the need for complex equalisers.
A. WiMAX uses OFDM-FDMA (frequency division multiple access). that this operation is conveniently performed by the IFFT (inverse fast Fourier transform). In particular. At the receiver.
Fig. the conclusions are given in Section VI. These types of LEDs have a phosphor layer on top of an InGaN-bases blue LED chip. the issue of high PAR in OFDM can be exploited constructively for visible light communication. As a consequence. 1 shows the principle block diagram of the demonstrator. is used. This type is chosen as it can be considered standard and inexpensive. I NTRODUCTION TO OFDM OFDM is considered a strong candidate as a wireless data transmission technology in broadband cellular networks. The optical source used in the prototype is a single chip (the ﬁrst type as described above) 5mm white LED with a luminous intensity of 11000mcd. the high signal variations of the time signal are utilised to intensity modulate the LEDs. the lighting will not be affected. it was decided to evaluate the performance using an experimental setup.
Visible light data transmission prototype
For the purpose of demonstration. The evaluation boards contain 32-bit stereo analogue input and output ports with a maximum sampling frequency of 96kHz [13]. and the possibility to easily combine OFDM with any higher order modulation scheme makes it an excellent choice also for visible light communication. Furthermore. Matlab/Simulink is used for the development of the algorithms and the code that is run on the DSP boards. B. The total channel bandwidth is divided into a number of orthogonal frequency subchannels. This is a particular problem when used for wireless transmission.16 based worldwide interoperability for microwave access (WiMAX) standard. which is based on the very long instruction word (VLIW) architecture. and SC-BPSK (sub-carrier binary phase shift keying) are some of the more popular modulation schemes used in conjunction with LED wireless systems [11]. the TMS320C6000 DSP evaluation board with the Texas Instruments C6713 ﬂoating-point processor. LED characteristics White LEDs are classiﬁed into two types. and. All the three colours are emitted simultaneously. simple off-the-shelf LEDs can be used to develop cheap transmitters. The link chain consists of two DSP boards. Moreover. PCM (pulse code modulation). The use of OFDM was ﬁrst discussed in [12]. Namely. In practical systems. Power ampliﬁers with a large linear range at reasonable costs are still an open issue.scribed in Section IV and Section V presents the measurement results. 1. At the receiver. a digital image is used as the data source. The generated D/A (digital/analogue) converted OFDM signal from the sender DSP is fed to the optical transmitter circuit that drives the white LED. Link chain Fig. H ARDWARE A RCHITECTURE Since no well deﬁned channel models for such transmission systems exist in literature. The other types of white LEDs are fabricated by mixing light from LEDs of the three primary colours. such as red. the possibility to combine it with any multiple access scheme such as TDMA (time division multiple access). OFDM is used in DVB (Digital Video Broadcasting) standards. II. Each low rate sub-stream is modulated on a separate subchannel. In particular. the power ampliﬁers have to be driven with a certain power back-off which compromises the signal coverage.

Concretely.8mm2 planar photodiode has a built in infrared rejection ﬁlter and provides a high shunt resistance of 0. some overprovisioning in the system design is accepted.
In addition. FEC coding is implemented. Root raised cosine pulse shaping ﬁlters with a rolloff of 0. IV. It is further assumed that the channel remains constant within one OFDM frame. On the OFDM Tx board. Frequency domain equalisation is realized using conventional OFDM zero-forcing (ZF) detection. This is based on a rate 1/2 convolutional encoder. With the particular implementation the high PAR in OFDM is exploited to intensity modulate the white LED. no well established models for this particular propagation system exist. A cyclic preﬁx of ﬁxed length is added to the transmitted signal. The
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Fig. ﬂat fading channel is assumed. The photodiode can be operated with an applied reverse bias.07GOhm maximum. and when it is used in a high gain transimpedance operational ampliﬁer circuit. Therefore.
the OFDM frame as implemented in the experimental system is formed by a time synchronisation signal (sinusoidal signal). 2.2 are used at the transmitter and receiver. The channel transfer factors are obtained using the training sequence and averaging over the four training sequence periods for every subcarrier. For the purpose of channel estimation and synchronisation.
Visible light OFDM transmission model
through the visible part of the spectrum. photovoltaic mode. only real valued signals can be transmitted. Viterbi decoding with hard decision output is used. The photodiode was chosen with the previously mentioned characteristics to achieve very low offset when the photodiode is operated in the photovoltaic mode. The data symbols are transmitted in the consecutive 20 OFDM symbols. four OFDM symbols for the training sequence. This is accomplished by dividing the OFDM symbol in two halves. A time interleaver is applied. In general a time varying. Each of these symbols uses four subcarriers for pilot transmission. For our application in the low frequency range. photoconductive mode. and low dark current of 2000pA maximum. pilots at speciﬁc subchannels are added to correct the residual channel estimation and synchronisation errors. V ISIBLE L IGHT S YSTEM M ODEL The main building blocks of an OFDM-based transmitter and receiver systems are illustrated in Fig. 3. 2. OFDM frame structure: Four OFDM symbols carrying a training sequence are used for channel estimation. the implementation on the DSP is straightforward. 15]. however. At the receiver. 3). or unbiased. Since OFDM is based on IFFT and FFT algorithms. Since.Pilots Binary-Rate Data Source Channel Encoder Interleaving Training QAMModulator
Assemble OFDM Symbol
OFDM Modulator
D/A Converter
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Binary Data
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Fig. and 20 OFDM symbols with data subchannels carrying the modulated information (see Fig. The generated photocurrent is proportional to the incident light power and it is converted to voltage using a transimpedance conﬁguration. The 9. With this approach the above made assumptions are to be conﬁrmed. training sequences and pilots are used [14. photovoltaic mode provides reasonable linearity and low noise.

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V. Measurements were taken for a distance up to 100cm. It is interesting to note. channel coding improves the BER performance to about 10−7 .
SNR measurements at the receiver
50cm can be achieved with QPSK modulation. 8 the magnitude and phase of the estimated optical channel for 14 consecutive OFDM frames are shown. This distance can hence be considered as the maximum possible distance of the single LED transmitter. as well as uncoded 64-QAM transmission are plotted.
Visible light demonstrator
The parameters of the OFDM transmission system are listed in Table I. The BER of OFDM for uncoded QPSK. the modulation is changed and the performance with and without channel coding is analysed. 7 shows the BER performance versus the distance between Tx and Rx units.] 98mm2 60 [deg. Fig.
TABLE I S YSTEM MODEL SPECIFICATIONS Number of FFT points Number of data sub-carriers Number of pilot sub-carriers Number of guard sub-carriers Number of guard interval samples Sampling frequency The O/E conversion efﬁciency at a receiver luminance intensity semi angle at half power of the transmitter Detector surface area FOV at the receiver 64 26 4 4 16 96kHz 0. The measurements are taken
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. and 26 subchannels for actual data transmission. The results show that already without any channel coding. 4.
Fig. Fig. the measurements were taken under ambient diffused sunlight inside a room. As the prototype is intended for indoor data transmission applications. and with the current setup a simplex link can be realised. The channel is estimated for each OFDM frame. In Fig. R ESULTS The demonstrator is depicted in Fig. 5. 6. As a consequence. 4. uncoded 16-QAM (quadrature amplitude modulation) transmission. A single LED is used.second half carries the conjugate complex copy of the ﬁrst half. 5. The OFDM symbols for data transmission are composed as follows (numbers in rows represent subcarrier indices): 4 pilot subchannels. a BER of about 10−4 at a Tx-Rx separation distance of
Fig. 6 shows the SNR measured at the receiver up to one meter. while the BER performance is still about 10−5 at 90cm Tx-Rx separation. For the coded case.
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Fig. These samples are fed into the root raised cosine pulse shaping ﬁlter and the resulting signal is then D/A converted. In order to study the performance of the system.53[A/W] 11000mcd 20 [deg. The measurements are based on a LOS (line-of-sight) channel. that the BER performance for coded QPSK transmission suddenly increases to 10−1 at a distance of about 1m. A further interesting observation is that at a distance of about 20cm (20% of the maximum distance). QPSK and 16-QAM results are shown. The conjugate complex of these 32 symbols (lower part) is transmitted on another 32 subchannels in order to generate a real valued signal for the intensity modulation of the LED. The time varying nature of the optical channel on the ﬁrst 30 non-zero sub-channels of an OFDM symbol can be observed. after the IFFT operation. For the same separation distance. The 14 OFDM frames are needed to transmit one complete TIFF (Tagged Image File Format) image ﬁle. 2 zeropadded subchannels to eliminate power at DC and at carrier frequency. The exact structure of an OFDM symbol is depicted in Fig. uncoded 64QAM still results in a reasonable BER performance. only real valued samples are obtained.